1. Field of the Invention
This invention relates in general to focus indicator systems for use with motion picture cameras. The invention relates specifically to a focus indicator system which uses parallax to indicate when an object in the field of view of a motion picture camera is in focus.
2. Description of the Related Art
Maintaining a properly focused image is critically important in film and television productions. This task is complicated by the fact that many shots require a moving object to be kept in focus or require different objects at different distances from the camera to be in focus at different times during the shot. Typically, on a movie set, the person responsible for focusing the camera, who is called the "first assistant camera" prepares for each shot by carefully measuring the distance to each object which will be in focus during the shot and then carefully marking the focusing ring on the camera lens with the measured distances. During the shot the first assistant camera adjusts the focusing ring to bring each object into precise focus at the desired time. This method works well with immobile objects. If the camera or the elements to be photographed are in motion it becomes very difficult to maintain the proper focus. Furthermore, this method is time consuming. The first assistant camera must make careful measurements before each shot. The enormous costs of production creates a need for a quick means for maintaining focus during a shot.
The prior art discloses a number of ways to maintain focus in a movie camera. U.S. Pat. No. 5,092,670 to Preston discloses an automatic focusing system for use with a motion picture camera. This system incorporates a laser range finder and triangulation to determine the distance to the object and adjusts the camera lens accordingly. The laser range finder is mounted on a tripod that is a fixed distance from the camera. While this system is useful for tracking moving objects, it is not practical for use with a moving camera. Where wide angle lenses are used in tight quarters it may be difficult to keep the range finder out of the field of view of the camera. Finally the system requires an operator for the range finder. This adds to the cost of filming.
Shenk, U.S. Pat. No. 4,531,822 discloses an auto-focussing camera incorporating an object presence detecting transducer. The camera automatically focuses on objects within its sonic acceptance angle. The Shenk device does not enable a user to focus on one of several objects within the acceptance angle or to shift the focus between different objects in the sonic acceptance angle.
Bogle et al., U.S. Pat. Nos. 4,601,557 and 4,534,629 disclose motion picture cameras having automatic focusing systems. The cameras uses microwave transmitters to transmit signals to targets wearing reactive signal generators. These systems require the objects of focus to wear a signal generator. This is not always practical, for instance when shooting nature documentaries.
Some prior art auto-focus systems include a partially silvered mirror, or the like, which intercepts some of the light passing through the lens of a motion picture camera. Such systems are undesirable because they degrade the optical path. In cinematography it is important to maintain the highest quality optical path possible.
One can compensate somewhat for inaccurate focussing by using a small lens aperture to increase the depth of field of the motion picture camera. This can be undesirable, especially when shooting at night, because a small lens aperture reduces the amount of light that passes through the lens. Using a small lens aperture can make it necessary to provide supplementary illumination. This adds expense and increases the time needed to set up for a shoot.
What is needed is a focus indicator system for a motion picture camera that will allow the person responsible for focusing the camera to quickly verify that a selected object is in precise focus. There is a particular need for such a device that can work with existing camera lenses and cameras.